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Namechk is an interesting web service that enables you to check the availability of your desired username on more than 120 websites including dozens of the most popular social networking and social bookmarking websites like Digg, Facebook, StumbleUpon and Twitter.
To check your username, simply type your prefer username in the text box and hit the “chk” button. This web service works really fast. It takes only a few seconds to check for available usernames.
Using namechk, you can ensure that your desired username is still available on the majority of the most popular sites.
Love this site! I use it often to check my own personal account name ideas and for clients.